The present disclosure is directed to toner compositions for use in electrostatographic or xerographic processes. More specifically, exemplary embodiments of the present disclosure relate to toner compositions comprising a friable external surface additive. It finds particular application in conjunction with toners and developers for electrostatographic processes, and will be described with particular reference thereto.
In electrophotography, a photoreceptor containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The photoreceptor is then exposed to a pattern of activating electromagnetic radiation, such as light. The radiation selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image. This electrostatic latent image may then be developed to form a visible image. In the development step, charged toner particles are deposited on the photoreceptor surface. There are several techniques by which this can be accomplished, such as charged-area-development (CAD) and discharged-area-development (DAD), most of which involve the use of a second component called a carrier.
In CAD, toner particles are attracted to the charged areas of the photoreceptor. This requires that the polarity of the toner particles be opposite to the photoreceptor surface potential. In DAD, the toner polarity is the same as the photoreceptor surface. As a result, the toner particles are repelled from the charged areas of the photoreceptor and deposit in the discharged regions. This requires the use of a developer housing bias with the same polarity as the photoreceptor surface potential.
By utilizing either process, the resulting visible image may then be transferred from the photoconductor to a support, such as transparency or paper. This imaging process may be repeated many times.
Various toner compositions for such imaging processes are well known in the art, and have been produced having a wide range of additives and constituent materials. Generally, however, the toner compositions or particles include a binding material such as a resin, a colorant such as a dye and/or a pigment, and any of various additives to provide particular properties to the toner particles.
The use of external additives in toner and developer compositions to improve a variety of characteristics in such compositions is known in the art. One type of additive that is commonly used in toner compositions is a surface additive. For example, surface additives may be incorporated to improve triboelectric charging behavior of the toners or developers, to improve the flow properties of the toner, and to improve development and transfer performance of the toner. An example of an external surface additive known in the art is hydrophobic fumed silica.
The primary particle size of surface additives, such as fumed silica, ranges from a few nanometers to tens of nanometers. However, these primary particles group together to form larger aggregates.
Developer compositions with charge enhancing additives, which impart a positive charge to the toner resin, are also known. For example, U.S. Pat. No. 3,893,935 describes the use of quaternary ammonium salts as charge control agents for electrostatic toner compositions. U.S. Pat. No. 4,221,856 discloses electrophotographic toners containing resin compatible quaternary ammonium compounds in which at least two R radicals are hydrocarbons having from 8 to about 22 carbon atoms, and each other R is a hydrogen or hydrocarbon radical with from 1 to about 8 carbon atoms, and A is an anion, for example sulfate, sulfonate, nitrate, borate, chlorate, and the halogens, such as iodide, chloride and bromide. Similar teachings are presented in U.S. Pat. Nos. 4,312,933 and 4,291,111. There is also described in U.S. Pat. No. 2,986,521 developer compositions comprised of toner resin particles coated with certain finely divided colloidal silica. According to the disclosure of this patent, the development of electrostatic latent images on negatively charged surfaces is accomplished by applying a developer composition having a positively charged triboelectric relationship with respect to the colloidal silica.
Also, there is disclosed in U.S. Pat. No. 4,338,390, the entire disclosure of which is incorporated herein by reference, developer compositions containing as charge enhancing additives organic sulfate and sulfonates, which additives can impart a positive charge to the toner composition. Further, there is disclosed in U.S. Pat. No. 4,298,672, the entire disclosure of which is incorporated herein by reference, positively charged toner compositions with resin particles and pigment particles, and as charge enhancing additives alkyl pyridinium compounds.
Additionally, other patents disclosing positively charged toner compositions with charge control additives include, for example, U.S. Pat. Nos. 3,944,493; 4,007,293; 4,079,014; 4,394,430 and 4,560,635, which illustrate a toner with a distearyl dimethyl ammonium methyl sulfate charge additive. Surface additives, such as silicas like AEROSILS™, may be incorporated into the toners of these patents.
Moreover, toner compositions with negative charge enhancing additives are known, as described, for example, in U.S. Pat. Nos. 4,411,974 and 4,206,064, the entire disclosures of which are incorporated herein by reference. The '974 patent discloses negatively charged toner compositions comprised of resin particles, pigment particles, and as a charge enhancing additive ortho-halo phenyl carboxylic acids. Similarly, there are disclosed in the '064 patent toner compositions with chromium, cobalt, and nickel complexes of salicylic acid as negative charge enhancing additives.
U.S. Pat. No.4,404,271 describes a toner that contains a metal complex where the metal can be chromium, cobalt or iron. Additionally, other patents disclosing various metal containing azo dyestuff structures wherein the metal is chromium or cobalt include U.S. Pat. Nos. 2,891,939, 2,871,233, 2,891,938, 2,933,489, 4,053,462 and 4,314,937. Also, in U.S. Pat. No. 4,433,040, the entire disclosure of which is incorporated herein by reference, there are illustrated toner compositions with chromium and cobalt complexes of azo dyes as negative charge enhancing additives. Other charge enhancing additives include those illustrated in U.S. Pat. Nos. 5,304,449, 4,904,762, and 5,223,368, the entire disclosures of which are incorporated herein by reference.
One of the problems associated with the use of external additives is that the improvements associated with the use of such additives may be lost when the additives become embedded into the toner particles surface as a result of mechanical forces encountered in the development hardware of the electrostatographic or xerographic machines. In high speed, high capacity systems, for example, long print runs with relatively low area coverage of color printing can cause the toner in the color developer housings to encounter long residence times and the resulting embedding of the external additives used in the system. As the external surface additives become embedded in the toner particle surface, the triboelectric and flow characteristics of the developer may decrease. This will result in poor development and poor transfer characteristics. Thus, embedding of the external additives into the toner particles decreases the effective life of the toner and/or developer, i.e., contributes to aging the toner and/or developer.
Attempts to solve the problem of toner aging in systems employing external additives have focused on a wide variety of parameters. Problems associated with the embedding of external additives into the toner surface have been addressed by selections of the type, size, and concentration of external additives used in toner and developer compositions. Attempts to reduce the aging of toners and developers have also focused on the blending techniques used to form toner compositions. The way in which an external surface additive is blended on the toner has a great effect on the performance of the system. If the blending is too gentle, the additive will not attach well to the toner surface, and the additive may collect in places that result an undesirable machine performance. Blending too aggressively, however, causes the additive to embed into the surface of the toner, which results in a loss of the beneficial effects of the external additive.
Despite the broad range of additives that have been used in formulating toner compositions, there is a continued need in the art for improved toner compositions that provide improved results and improved image quality. Consequently, it is desirable to provide a toner composition that will not suffer from the detrimental effects associated with conventional external additives that result from external additive particles embedding into toner particles. It is desirable to provide a toner and/or developer composition that has a longer life as compared to toners or developers using conventional external additives. It is also desirable to provide a toner employing an external additive that does not exhibit high relative humidity sensitivity. It is further desirable to provide a toner that is more robust relative to previous toners with respect to aging in developer housings due to being subjected to mechanical forces. It is still further desirable to provide an additive for a toner, and a toner using such an additive, that offers improved flow properties, prevents agglomeration, and extends the life of the toner while providing satisfactory triboelectric characteristics to the toner.